Low voltage protection circuit for induction motors

Abstract

A low voltage protection circuit for an air conditioner compressor which employs a series of diodes to control the application of an a.c. source potential to the resistive heater element of a conventional thermally responsive switch. The diode circuit maintains the heater element directly across the source potential at all times when the thermally responsive switch is open, but only during alternate half-cycles of the source potential when the thermally responsive switch is closed to thereby protect the compressor against potentially damaging low voltage conditions while reducing the power consumed by the heater element during periods of normal source voltage level.

I claim: 
     
       1. In an improved low voltage protection circuit for an a.c. motor of the type comprising first switching means for applying a source of a.c. potential across said motor when activated, and   resistance means operatively associated with said first switching means for activating said first switching means when a current through said resistance means is at least equal to a minimum value, the improvement of which comprises second switching means including:   a first pair of diodes connected to and on each side of said resistance means, the resulting series combination of which is connected across the series combination of said motor and first switching means, for passing a current through said resistance means during each half-cycle of a first polarity of said source potential, and   a second pair of diodes connected to and on each side of said resistance means, the resulting series combination of which is connected across said first switching means for passing a current through said resistance means during each half-cycle of a second polarity of said source potential when said first switching means is inactive, said second pair of diodes and said resistance means being shunted out of said circuit by said first switching means when the latter is activated.   
     
     
       2. The circuit of claim 1 further comprising a manually operable on/off switch for applying said source potential to said motor, and   a thermostatically controlled switch for applying said source potential to said motor, said motor being an air conditioner compressor.   
     
     
       3. A low voltage protection circuit for an air conditioner compressor comprising a manually operable on/off switch,   an adjustable thermostatically controlled on/off switch,   a normally open heat activated switch, said switches being connected in series with said compressor for controlling the application of an a.c. source potential across said compressor,   a heater element thermally coupled to said heat activated switch for generating heat in response to a current flow through said element to close said heat activated switch when said current is at least equal to a preselected minimum value, and   diode switching means for connecting said element across said source potential at all times when said heat activated switch is open and for connecting said element across said source potential only during alternate half-cycles of said source potential when said heat activated switch is closed.